linux-kernel - Re: [patch] mm, slub: ensure irqs are enabled for kmemcheck

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Message-ID: <20120709140622.GA26595@localhost>
Date:	Mon, 9 Jul 2012 22:06:22 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	JoonSoo Kim <js1304@...il.com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Christoph Lameter <cl@...ux.com>, Rus <rus@...nxsoft.com>,
	Ben Hutchings <ben@...adent.org.uk>, stable@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] mm, slub: ensure irqs are enabled for kmemcheck

On Mon, Jul 09, 2012 at 09:46:33AM -0400, Steven Rostedt wrote:
> On Mon, 2012-07-09 at 03:36 -0700, David Rientjes wrote:
> > kmemcheck_alloc_shadow() requires irqs to be enabled, so wait to disable
> > them until after its called for __GFP_WAIT allocations.
> > 
> > This fixes a warning for such allocations:
> > 
> > 	WARNING: at kernel/lockdep.c:2739 lockdep_trace_alloc+0x14e/0x1c0()
> > 
> > Cc: stable@...r.kernel.org [3.1+]
> > Acked-by: Fengguang Wu <fengguang.wu@...el.com>
> > Tested-by: Fengguang Wu <fengguang.wu@...el.com>
> > Signed-off-by: David Rientjes <rientjes@...gle.com>
> > ---
> >  mm/slub.c |   13 ++++++-------
> >  1 file changed, 6 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -1314,13 +1314,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> >  			stat(s, ORDER_FALLBACK);
> >  	}
> >  
> > -	if (flags & __GFP_WAIT)
> > -		local_irq_disable();
> > -
> > -	if (!page)
> > -		return NULL;
> > -
> > -	if (kmemcheck_enabled
> > +	if (page && kmemcheck_enabled
> 
> One micro-optimization nit...
> 
> If kmemcheck_enabled is mostly false, and page is mostly true, wouldn't
> it be better to swap the two?
> 
> 	if (kmemcheck_enabled && page
> 
> Then the first check would just short-circuit out and we don't do the
> double check.

I had the same gut feeling but at the time was not as conscious as you ;)
Now I can dig out a similar optimization by Andrew Morton which also
saves memory bytes:

On Tue, Jun 19, 2012 at 03:00:14PM -0700, Andrew Morton wrote:

: With my gcc and CONFIG_CGROUP_MEM_RES_CTLR=n (for gawd's sake can we
: please rename this to CONFIG_MEMCG?), this:
: 
: --- a/mm/vmscan.c~memcg-prevent-from-oom-with-too-many-dirty-pages-fix
: +++ a/mm/vmscan.c
: @@ -726,8 +726,8 @@ static unsigned long shrink_page_list(st
:                          * writeback from reclaim and there is nothing else to
:                          * reclaim.
:                          */
: -                       if (PageReclaim(page)
: -                                       && may_enter_fs && !global_reclaim(sc))
: +                       if (!global_reclaim(sc) && PageReclaim(page) &&
: +                                       may_enter_fs)
:                                 wait_on_page_writeback(page);
:                         else {
:                                 nr_writeback++;
: 
: 
: reduces vmscan.o's .text by 48 bytes(!).  Because the compiler can
: avoid generating any code for PageReclaim() and perhaps the
: may_enter_fs test.  Because global_reclaim() evaluates to constant
: true.  Do you think that's an improvement?

Thanks,
Fengguang
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